Currently this project focusses on a model system of nuclear differentiation in Paramecium. Alternative nuclear differentiations determine the capacity of trichocysts to discharge or not discharge from intact cells of identical genotype in response to picric or tannic acid. Determination of these nuclear differentiations occurs only during the first 20 hours following fertilization and is thereafer clonally stable. The investigation will explore external and internal factors operative in bringing about nuclear differentiation during the sensitive period and their mode of action; and the nuclear and cytoplasmic differences distinguishing cells of identical genotype that are in the two alternative stable, hereditary conditions.